some forms of life flourish in this salty environment. but humans require an abundant supply of freshwater. less than 1% of all the earth's water is readily available for human consumption. in the semi-arid desert plains of the southwest united states, where rainfall averages just 2 1/2 centimeters per month, tom maddock studies this scarce resource. dr. maddock: the real problem that we have is that with increasing populations and shortages of water, we are becoming very vulnerable. in the southwest, there's a very unique vulnerability here, simply because where do we get the water if there is no water? narrator: across the country in northern florida, the quantity of water isn't an issue. rainfall averages an abundant 1 1/4 meters each year. wendy graham and her colleagues evaluate and model the impacts of industrial and agricultural land use threatening the world's largest collection of freshwater springs. dr. graham: right now, the biggest question is how far we can stress the system and not push it past the point of no return. narrat